[vm] Do not mirror default values of parameters of abstract methods

In bytecode mode default values are part of the method body, which is
omitted for abstract methods.

In anticipation for this future change, default values of parameters of
abstract methods are no longer available through dart:mirrors.

Please also see https://github.com/dart-lang/sdk/issues/38464

Change-Id: I47742b588690ea96cb3ca636ff86e4e042bfe5a2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117299
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

CHANGELOG.md

@@ -20,6 +20,9 @@ main() { foo(() {}); }
 
 ### Core libraries
 
+* Default values of parameters of abstract methods are no longer available
+  via `dart:mirrors`.
+
 ### Dart VM
 
 ### Tools

runtime/vm/kernel.cc

@@ -571,7 +571,7 @@ class ParameterDescriptorBuilder : public KernelReaderHelper {
         type_translator_(this, active_class, /* finalize= */ true),
         constant_evaluator_(this, &type_translator_, active_class, nullptr) {}
 
-  RawObject* BuildParameterDescriptor(intptr_t kernel_offset);
+  RawObject* BuildParameterDescriptor(const Function& function);
 
  private:
   TypeTranslator type_translator_;
@@ -581,8 +581,8 @@ class ParameterDescriptorBuilder : public KernelReaderHelper {
 };
 
 RawObject* ParameterDescriptorBuilder::BuildParameterDescriptor(
-    intptr_t kernel_offset) {
-  SetOffset(kernel_offset);
+    const Function& function) {
+  SetOffset(function.kernel_offset());
   ReadUntilFunctionNode();
   FunctionNodeHelper function_node_helper(this);
   function_node_helper.ReadUntilExcluding(
@@ -610,17 +610,19 @@ RawObject* ParameterDescriptorBuilder::BuildParameterDescriptor(
                            helper.IsFinal() ? Bool::True() : Bool::False());
 
     Tag tag = ReadTag();  // read (first part of) initializer.
-    if (tag == kSomething) {
+    if ((tag == kSomething) && !function.is_abstract()) {
       // this will (potentially) read the initializer, but reset the position.
       Instance& constant = Instance::ZoneHandle(
           zone_, constant_evaluator_.EvaluateExpression(ReaderOffset()));
-      SkipExpression();  // read (actual) initializer.
       param_descriptor.SetAt(entry_start + Parser::kParameterDefaultValueOffset,
                              constant);
     } else {
       param_descriptor.SetAt(entry_start + Parser::kParameterDefaultValueOffset,
                              Object::null_instance());
     }
+    if (tag == kSomething) {
+      SkipExpression();  // read (actual) initializer.
+    }
 
     if (FLAG_enable_mirrors && (helper.annotation_count_ > 0)) {
       AlternativeReadingScope alt(&reader_, param_kernel_offset);
@@ -665,7 +667,7 @@ RawObject* BuildParameterDescriptor(const Function& function) {
         ExternalTypedData::Handle(zone, function.KernelData()),
         function.KernelDataProgramOffset(), &active_class);
 
-    return builder.BuildParameterDescriptor(function.kernel_offset());
+    return builder.BuildParameterDescriptor(function);
   } else {
     return Thread::Current()->StealStickyError();
   }

tests/lib_2/mirrors/parameter_abstract_test.dart

@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// BOGUS: Note that both AST and bytecode modes are missing default values.
-
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
@@ -23,12 +21,12 @@ main() {
   MethodMirror foo1 = cm.declarations[#foo1];
   expect('Method(s(foo1) in s(C), abstract)', foo1);
   expect(
-      'Parameter(s(x) in s(foo1), optional, named, value = Instance(value = 1), type = Class(s(int) in s(dart.core), top-level))',
+      'Parameter(s(x) in s(foo1), optional, named, type = Class(s(int) in s(dart.core), top-level))',
       foo1.parameters[0]);
   expect(
-      'Parameter(s(y) in s(foo1), optional, named, value = Instance(value = 2), type = Class(s(int) in s(dart.core), top-level))',
+      'Parameter(s(y) in s(foo1), optional, named, type = Class(s(int) in s(dart.core), top-level))',
       foo1.parameters[1]);
   expect(
-      'Parameter(s(z) in s(foo1), optional, named, value = Instance(value = 3), type = Class(s(int) in s(dart.core), top-level))',
+      'Parameter(s(z) in s(foo1), optional, named, type = Class(s(int) in s(dart.core), top-level))',
       foo1.parameters[2]);
 }

tests/lib_2/mirrors/stringify.dart

@@ -175,7 +175,7 @@ String stringify(value) {
   throw 'Unexpected value: $value';
 }
 
-expect(expected, actual, [String reason]) {
+expect(expected, actual, [String reason = ""]) {
   Expect.stringEquals(expected, stringify(actual), reason);
 }